Much attention has been paid by the paper industry to chemically pre-treating cellulosic suspensions for the purpose of improving the retention and drainage properties thereof. For example, it is common to include various inorganic materials, such as bentonite and alum, and/or cationic organic materials, such as natural or modified natural or synthetic polymers, in the thin stock for the purpose of improving the papermaking process. These additives are used for pitch control, decoloration of the drainage water or for facilitating release from drying rolls. Starch is often included to improve strength.
As discussed in U.S. Pat. No. 4,753,710 (Langley et al.), which issued on Jun. 28, 1988, paper stocks may have both an inorganic additive and an organic polymeric material for the purpose of improving retention, drainage, drying and/or formation. For example, a stock may include bentonite, an aluminum sulfate coagulant, and a cationic polymer such as polyethylene imine to improve dewatering. Others have treated paper stock with a filler, a nonionic polyacrylamide, and bentonite. Still others have demonstrated that addition of either a cationic starch or cationic polyacrylamide and bentonite also improves retention. Another process which is believed to result in a suspension having good strength and satisfactory retention includes colloidal silicic acid and cationic starch additives.
In particular, U.S. Pat. No. 4,753,710 provides for the addition of an inorganic material such as bentonite after one of the shear stages, and an organic polymeric material such as a substantially linear, synthetic, cationic polymer (e.g., a cationic polymer flocculant) having a molecular weight above 500,000 and which is added to the suspension before the shear stage in an amount which is at least 0.03%, based on the dry weight of the suspension. It is also common to include a filler, such as calcium carbonate, clay, titanium dioxide or talc or a combination, in the cellulose suspension or paper stock. The filler is preferably incorporated into the stock before addition of the synthetic polymer.
The stock may include other additives such as rosin, alum, neutral sizes or optical brightening agents. It may also include a strengthening agent and this can be a starch, often a cationic starch. The pH of the stock is generally in the range of 4 to 9.
An improvement of U.S. Pat. No. 4,753,710 is disclosed in European Patent Publication No. 0 335 575 (Langley), which was published on Oct. 4, 1989. This patent application was directed primarily to newsprint and board, wherein a low molecular weight cationic polymer, e.g., polyethylene imine, polyamines, polycyandiamide formaldehyde polymers, amphoteric polymers, and polymers of monomers selected from diallyl dimethyl ammonium chloride, diallylaminoalkyl(meth) acrylates and dialkylaminoalkyl(meth) acrylamides, is added to the fiber suspension, followed by addition of a starch, followed by the addition of bentonite or colloidal silicic acid after the shear stage.
The chemical treatment program of a cationic coagulant/cationic flocculant/bentonite program disclosed in European Patent Publication No. 0 335 575 creates problems due to the handling of dry bentonite, a known carcinogen, which is extremely harmful. Therefore, it would be most desirable to develop alternatives to dry bentonite. The present inventor has developed a novel liquid slurry of bentonite which overcomes the problems associated with dry bentonite, i.e., health risks and effect of aging on swellability. The novel liquid slurry of bentonite is pumpable and is readily dispersed in aqueous solution with minimum mixing, i.e., no handling of the dry bentonite is required. Also, the performance of bentonite is closely related to its swellability (i.e., aging effect). The novel liquid slurry of bentonite produces a stabilized bentonite product without materially effecting its performance.
The present invention also provides many additional advantages which shall become apparent as described below.